The field of the invention is in the interferometer art and more particularly that of a interferometer-type device for detecting and analyzing coherent radiation (e.g. a laser beam) in the presence of non-coherent radiation.
Interferometers have been known for over one hundred years. Many types of interferometers such as the Michelson, the Fabry-Perot, and the Fizeau have been well developed. Interferometers have been widely used to measure both very small and very large distances and fur the measurement of wavelengths and frequencies. Generally, optical interferometers function through the combining of two separate rays of collimated monochromatic light, phase adjusted by a movable element, to provide a fringe pattern. Before the advent of the laser, the obtaining of sufficiently coherent light for a light source was frequently a problem.
The use of the interferometer to detect and analyze laser emissions is well known. For example, U.S. Pat. No. 4,172,663 to Byer et al discloses a plurality of Fabry-Perot etalon interferometers with the light input to and the light outputs from the etalons, focused by lenses, and sequentially directed onto a spatial detector; while a device having delay, shift, and, lead is disclosed for obtaining the wavelength of laser emissions, the angle of arrival information is not produced. Crane in U.S. Pat. No. 3,824,018 rotates a Fabry-Perot etalon through various angles of incidence perpendicular to the optical axis of the etalon to effect a simultaneous two-dimensional scan of the object space, thus providing directional information of a received laser beam. U.S. Pat. No. 4,173,442 to Snyder teaches the use of a Fizeau-type interferometer wherein "tilt" is introduced and a parallel fringe pattern provided to determine the wavelength of light from a monochromatic source; but again, no information is provided regarding the position or direction of the source.
Richard Hartman in the publication of "Defence Electronics" for May 1979 at pages 79-85 describes a Fabry-Perot sensor with a stepped etalon for detecting laser radiation. U.S. Pat. No. 4,170,416 to Fencil discloses a four-quadrant Fabry-Perot etalon optically modulated by tilting or angularly moving the entire unit to distinguish between coherent and non-coherent sources and to obtain the direction of the coherent sources.
Generally, the prior art devices that will detect, analyze, and indicate direction of a coherent beam in the presence of non-coherent radiation involve a moving element, e.g., scanning in the interferometer. Such devices are not rugged, they are complex to fabricate, frequently subject to blind spots, generally have limited acceptance angles, and are quite expensive.